Whenever you fill your (gasoline-fueled) car, you have choices—one of them being premium. What is this high-test fuel anyway? Is it required for your car? Merely recommended? Or only a waste of money?

With significant changes occurring in automotive engineering, are we going to see increased specification of this more costly fuel? Or will these refinements of internal combustion make premium less likely a choice?

To answer these, I chatted with specialists at Honda, Mercedes-Benz, Afton Chemical and our fuel partners Shell. I also consulted 2012 auto press kits, Ethyl: A History of the Corporation and the People Who Made It, my trusty Bosch Automotive Handbook and Van Basshuysen and Schäfer's even more exhaustive Internal Combustion Engine Handbook.

Let's begin with a charming story that may even be true. In the early days of motoring, a salesman peddling wares to a fuel supplier would adjust his Model T Ford's spark advance lever until the engine was pinging like crazy. Then he'd sprinkle a mysterious liquid on his tie, wave the tie past the Model T's carburetor intake—and the pinging would instantly diminish.

Don't try this at home, kids; he was a professional—and the mysterious stuff was tetraethyl lead (TEL). TEL was the world's best anti-knock additive. But it's also extremely toxic and no longer used to prevent knock in automotive engines. Factoid: In 1924, a spate of lead poisoning and subsequent insanity among chemical workers led to TEL-treated fuels being called "loony gas." It can also be noted that TEL's phaseout in the 1970s was not directly focused on health concerns, but rather on its poisoning of catalytic converters.

I Hear You Knocking

Also known as detonation and, in its lesser form as ping, knock is uncontrolled combustion occurring ahead of the sparkplug-initiated flame front. Trace ping is annoying rather than harmful; heavy knock, especially at high revs, can quickly destroy a piston. Neither is indicative of MBT, Minimum advance for Best Torque, the optimal ignition setting for a particular combination of engine speed, load—and fuel.

These days, our engines' electronic control units select optimal timing. (And I shudder to think what many modern drivers would do with a manual spark advance lever.) What's more, modern engines have knock sensors listening for incipient ping. If detected, they dial back ever so slightly on ignition timing to eliminate it even before it's audible to the likes of us.

Octane is Knock Resistance

And if you're using the fuel recommended by the automaker, that knock sensor rarely has anything to detect. Pure and simple, a gasoline's octane rating is a measure of its resistance to knock as compared to a particular pair of hydrocarbons. Isooctane, an especially knock-resistant variety, is 100 on the octane scale; at the other extreme, n-Heptane is 0. Two procedures with differing test conditions determine a gasoline's Motor Octane Number and its Research Octane Number. The average (MON + RON)/2 is the one we call "pump octane" displayed on the service station pump.

Our premium fuels typically have pump octanes of 91–93. Regulars are 87 octane; midgrades, 89. (Why a midgrade? We'll get to this in a moment.) And, of course, befitting its name, premium fuels come at a premium price, currently in our region 20¢/gal. beyond regular.

The simplest answer of which fuel to use is to follow automaker advice contained in your car's owners manual. This might be given as a requirement or merely as a recommendation. The latter might be qualified by a statement along the lines of "lower octane may yield reduced performance or inferior mpg."

The reason for this has nothing to do with energy content. In fact, depending on its blending, a premium fuel may actually contain less energy per unit volume than regular. However, its potential for producing more power and enhanced mpg goes back to MBT and that knock sensor. If premium is recommended, this is the engine's optimal fuel. It'll run on regular—albeit with the sensor invoking ignition timing that's less than that associated with MBT. And with less than Best Torque, there'll be less performance and an mpg hit.

If the engine is tuned for regular but fueled with something of higher octane, things are a tad more complex. Most modern knock-sensed ignitions seek MBT timing and thus, at least in theory, profit from the added octane. Some, though, have preset ceilings beyond which they won't advance.

And, in all cases, the differences depend on engine families, their octane sensitivities and intended performance levels. Without a stopwatch or dyno—and apart from your pocketbook—you may not even notice any detriment or benefit. By the way, in R&T performance testing at the track, we arrange matters to use Shell's locally available premium of 91 pump octane.

What of midgrade? There's logic involving "legacy vehicles," older high-mileage machinery designed for regular. Even in these days of unleaded fuel, other combustion byproducts can cause deposits in the chamber that may increase octane appetite just a bit. The added octane of midgrade (blended at the pump from the station's premium and regular) might be just enough to counter this.

Refinery Aspects

Refineries take crude petroleum and transform it into a wealth of products, everything from tar and bunker fuel to middle distillates like diesel, jet fuel and kerosene to high-end products like gasoline. And a petroleum engineer's life would be a lot simpler if the crudes were uniform in their makeup.

Alas, they're not. Some crudes, like Sweet Bonny Light from Nigeria, have especially low sulfur. Others, from Yucatán, for instance, are sour. Refineries are optimized for particular inputs and specialized products; the newer facilities, rather more adaptable. On average, a 42-gal. barrel of crude can yield about 19.5 gal. of gasoline, about 46 percent.

Premium gasolines get their enhanced octane in a variety of ways. Aromatic hydrocarbons inherently have higher octane. However, health limits on these—benzene, in particular—give rise to other refinery practices. For instance, catalytic cracking breaks larger hydrocarbons into smaller bits, octane (C8H18) being one of these. Isomerization re­arranges knock-prone straight-chain molecules into branched isomers.

Ethanol, used as an extender in much of our gasoline, is also an octane enhancer. Blended at a minimum of around 6 percent, its pump stickers state "up to 10 percent ethanol" to account for refinery optimizations of catalytic cracking, isomerizing, alcohol enhancement and other techniques.

Additive packages enhance the properties of gasolines. Detergents reduce carbon buildup, all the more important these days with high-pressure direct-injection hardware. Antioxidants improve the storage and tank life. Corrosion inhibitors and dyes are also added. It's generally suggested—and no suppliers deny—that premium fuels have more extensive (spelled $$$) additive packages.

Other Premium Nuggets

As already noted, premium gasoline may actually contain less specific energy than regular, though this is more than countered by exploitation of its higher octane. This is not the case with ethanol, however, nor with E85, its 85-percent blending with 15-percent gasoline. E85's pump octane of 94Ω96 isn't enough to counter its paucity of energy—around 70 percent of gasoline's—and EPA comparisons of flexible-fuel mpgs confirm this.

Another interesting factoid, albeit one that's a bit contorted: Premium fuel is good for a catalytic converter. In allowing advanced timing of ignition, the fuel has a longer burn duration. This in turn gives more time for heat transfer within the combustion chamber—which results in reduced temperature of exhaust gases. This cooler "engine-out" condition makes for an easier thermal life of downstream catalysts. (It's a carefully engineered balance—quick warmup to cut cold-start emissions, hot enough to promote catalyst efficiency, but not to the detriment of durability.)

What of the Future?

Despite hype suggesting otherwise, the era of internal combustion is far from over. What's more, efficiency enhancements such as higher compression ratios and forced induction profit from more octane.

However, marketing considerations come into play as well. Manufacturers of high-end or high-performance cars can specify premium without a pause; their buyers expect nothing less. By contrast, full-range automakers are more likely to engineer appropriate entries in their lineup for regular. Last, everyone has to account for owners who ignore fuel recommendations, though, as one specialist put it, "no one wants to live off the knock sensor."

Mazda's new SkyActiv engine family is exemplary of the trend for higher efficiency and also of marketing considerations. Direct injection, artful design of piston geometry and other nuances give our Mazda3's SkyActiv powerplant an octane appetite for regular; this, despite its 13.0:1 compression ratio. The Euro version gets an even loftier 14.0:1 with commensurate enhancements of power and efficiency—but also with a premium fuel requirement unwarranted for the North American market.

The Chevrolet Volt and its Cruze Ecosibling present another interesting case. The Volt's 1.4-liter 4-cylinder gasoline engine has primarily a series hybrid role; on demand, the engine fires up to run a generator supplying the car's electric propulsion needs. This engine produces 84 bhp—and requires premium fuel. A 10.5:1 compression ratio and commensurately more spark advance are said to enhance efficiency to the tune of 5–10 percent better fuel economy. Another payoff is premium's more extensive additive package, among them more antioxidant improving the gasoline's tank life—important if the car is only infrequently run beyond its battery range.

By contrast, the Cruze Eco uses a variant of this dohc inline-4. Only this one has a turbo, a 9.5:1 compression ratio—and produces 138 bhp on regular.

Clearly there are multiple paths to automotive efficiency—and one of them seems destined to be premium-fueled.